238 Domestic Science 



thermometer, the bulb of which is surrounded by loose 

 strands of lamp- wick, the other ends of which dip 

 into a vessel which is kept nearly filled with water. 

 By means of this arrangement the bulb of the latter, 

 instrument remains continually moist. 



Take the readings of both at as nearly as possible 

 the same instant. It will be found that the temperature 

 recorded by the dry bulb thermometer is higher than 

 that shown by the other. The explanation of this 

 difference is as follows : 



Water is always evaporating from the threads in 

 contact with the bulb of the latter at a rate which 

 depends on the dryness of the air around the bulb. 

 We have already shown in Chapter x that when 

 evaporation occurs, heat is absorbed. In the case of 

 the hygrometer, there is no supply of heat from an 

 outside source which can compensate for the heat thus 

 absorbed, and hence the heat necessary is supplied by 

 the objects in contact with the water which is being 

 vaporised, i.e. by the bulb of the thermometer. As a 

 result, the latter is cooled below the temperature of 

 the room, and hence the wet bulb thermometer registers 

 a lower temperature than its fellow. 



On a very dry day, this difference may be consider- 

 able, while if the atmosphere be very moist, it may 

 amount to less than 1 C. 



A series of readings of the wet and dry bulb ther- 

 mometers should be taken daily at the same time as 

 the weighing of the beaker of water in Experiment 99. 

 The average differences of temperature during the 

 intervals between the times of reading may be assumed 

 to be approximately the mean of each pair of consecutive 

 difference readings. 



On the same sheet of squared paper as used in 



